An attractive and logical treatment strategy for human genetic disorders resulting from a single gene defect is direct protein/enzyme replacement. We are recombinantly producing human proteins/enzymes, some of which can be further modified to enhance their clinical effectiveness. Recombinant human proteins that are under study include lysosomal enzymes (glucocerebrosidase and alpha-galactosidase), neurotransmitter synthesizing enzymes (tyrosine hydroxylase and tryptophan hydroxylase) and other proteins (saposin activator proteins). Enzyme replacement therapy for Gaucher disease with Alglucerase has been shown to be clinically effective, but its extremely high cost is of considerable concern both to the public and to health care providers. As a potential alternative therapy to Alglucerase, we have produced large amounts of recombinant human glucocerebrosidase using a baculovirus expression system and have chemically modified the enzyme with Polyethylene Glycol (PEG) to increase plasma survival. This chemical modification may potentially offer significant benefit to patients by decreasing drug cost, antigenicity, and dosage, and also by providing a more convenient route of administration, such as intramuscular or subcutaneous delivery. A Phase I drug study testing the safety, pharmacodynamics, pharmacokinetics and efficacy of PEG-glucocerebrosidase was begun. Recombinant active proteins/enzymes are also being produced in the milk of transgenic animals. Different DNA constructs containing the human glucocerebrosidase gene and a variety of mammary gland promoter sequences have been used to create transgenic mice which express high levels of human glucocerebrosidase in their milk. These constructs are now being optimized and tested in pigs to enable the production of large quantities of therapeutically useful proteins.